To see how the CF-to-IDE-to-SCSI solution really performs, I tried it in a slightly faster system. GA-586HX motherboard (Intel 430HX chipset), AMD K5-133 processor, and PCI SCSI HBAs.
The first tested configuration was using a Tekram DC-390 HBA. Sysinfo showed 8.7 MB/s, not much more than the 8.4 MB/s the VLB 486 reached. The limit here is probably the 10 MB/s Fast SCSI transfer rate and we’re getting close to 90% of the theoretical maximum, although a different HBA would perhaps do better. The DC-390 is a Fast SCSI HBA limited to 8-bit wide transfers and 10 MB/s.
The second HBA was a classic Adaptec AHA-2940UW, but it did slightly worse than the Tekram, at a little under 8.6 MB/s. But wait, the ‘U’ in 2940UW stands for Ultra, and the Acard IDE-to-SCSI adapter is the 7720U, where ‘U’ once again indicates Ultra SCSI. So why can’t we get past 10 MB/s?
Because the 2940UW does not enable Ultra SCSI by default. And sure enough, enabling it bumped up the speed a little: 10.6 MB/s. That’s not a lot better but clearly we’re past 10 MB/s. At this point it’s unclear what the bottleneck is. It could be the PCI bus, host board, CPU, DOS, the HBA’s BIOS, the CF card, or perhaps something else. The system should have sufficient memory bandwidth (about 60 MB/s write throughput) and BIOS ROM access should not be an issue on a PCI board (it’s all copied into RAM anyway).
The system feels snappier than when the same CF card (a 1.0 GB SanDisk Ultra II) is attached to IDE directly. Sadly I find myself unable to compare the speed with Sysinfo due to geometry problems (an odd issue that I won’t go into now), namely Sysinfo refusing to benchmark the disk when it’s attached to IDE.
So let’s try a different card, a 2.0 GB SanDisk Ultra III. It does rather well with IDE, showing 9.2 MB/s on this motherboard. But whoops… Sysinfo fails again, this time not because it can’t see the disk but because it shows 0.0 MB/s when it’s attached via SCSI!
So let’s try Speedsys 4.78. For the SanDisk Ultra III CF card attached via IDE, it shows 8,579 KB/s buffered read speed and 8,372 KB/s linear read speed. When attached via SCSI, the values jump up to 13,310 KB/s buffered read speed and 11,752 KB/s linear read speed. It’s ironic that CF-to-IDE-to-SCSI is faster than a direct CF-to-IDE connection.
With PC-CONFIG V8.20, the figures are as follows. When using direct CF-to-IDE connection, PC-CONFIG reports 11,532 KB/s linear read speed and max throughput of 8,851 KB/s. Going CF-to-IDE-to-SCSI, we get linear read speed 23,064 KB/s, max throughput 13,519 MB/s. The throughput is presumably the more important figure. 13.5 MB/s is not at all bad for a Pentium system; and again it’s noticeably better than the same CF card via IDE.
In this configuration, it makes a big difference what kind of CF card (or Microdrive) is plugged into the IDE-to-SCSI adapter. Many CF cards can’t achieve such speeds and top out at 5 MB/s or less.
The conclusion is that the CF-to-IDE-to-SCSI solution performs quite well, and a relatively fast CF card is required to get past 10 MB/s. For vintage systems the performance is more than adequate.
Apologies for the lack of photos in this post.